Thapsigargin (TG) is a sesquiterpen lactone that inhibits the endoplasmic reticulum (ER) calcium ATPases to disrupt calcium homeostasis and consequently induces ER stress. We have previously reported that TG induces apoptosis by engaging the death receptor 5 (DR5) and the intrinsic pathways.  Second mitochondrial-derived activator (Smac) is an important modulator of apoptosis that induces activation of caspases by antagonizing inhibitors of apoptosis (IAPs).  In this study, we have utilized Smac-proficient and -deficient human colon cancer cells to investigate the effects of Smac deficiency during ER-stress-induced apoptosis.  Our results indicate that Smac deficiency considerably affects ER stress-induced apoptosis in human colon cancer cells.  For example, ER stress inducing agent TG upregulates DR5, and activates caspases 3, 9 and 8 in Smac-proficient cells.  In Smac-deficient cells, although TG-induced DR5 upregulation is not affected, activation of caspases 3, 9 and 8 is affected.  Smac deficiency also affects TG-induced cytochrome c release from mitochondria into cytosol suggesting the existence of a potential cross-talk between Smac and cytochrome c.  Thus, our results indicate that ER stress-induced apoptosis also engages Smac for transduction of apoptotic signals in human colon cancer cells and that a potential feedback signaling between Smac and cytochrome c appears to modulate the intrinsic pathway of apoptosis.